This invention is directed toward avalanche photodetectors (APDs) useful in high-speed optical fiber applications. More particularly, the invention is related to avalanche photodetectors having high and controllable electric fields in both their absorption and multiplication regions.
Optical fiber transmission systems are being increasingly deployed to meet the demand for high throughput telecommunication and data signal transmission. With the increasing use of optical fiber transmission systems there is a concomitant need for cost-effective high performance (i.e., low noise, high speed) components to interface between the optical fiber used for signal transmission and receiver electronics.
Interface components such as optical detectors used with the optical fiber systems are often based on the InGaAs/InP material system. InGaAs exhibits material properties such as high absorption of light in the wavelength range between 1.3 xcexcm and 1.5 xcexcm, high carrier mobilities and saturation velocities. These properties make InGaAs a material of choice for making efficient, high-speed photodiodes. InGaAs layers are conveniently grown on InP substrates using common epitaxial growth techniques. Most of today""s APDs use InGaAs as the material for the absorption region, and use InP as the material for the multiplication region. The avalanche multiplication gain and hence the overall gain-bandwidth performance of an APD depends on the difference between the electron and hole ionization coefficients of the material used for the multiplication region. For InP material these coefficients are nearly the same. Therefore, the gain-bandwidth product of InGaAs/InP APDs is limited.
Materials such as silicon which have dissimilar electron and hole ionization coefficients are more suitable than InP for obtaining high multiplication gain. Silicon is indeed used for making low-noise and high-speed operation APDs, for example, high-performance reach-through APDs. However, silicon does not absorb light in the optical wavelength regions used for optical fiber signal transmission and cannot be used as the absorption region material for light in the wavelength range between 1.3 xcexcm and 1.5 xcexcm.
Recently, attempts have been made to combine the desirable optical properties of InGaAs material with the desirable low-noise and high-speed properties of silicon. Using wafer fusion techniques, composite InGaAs/silicon mesa-type APDs have been demonstrated. In these APDs an InGaAs/InP substrate wafer is fused on top of a silicon wafer. The InP substrate is etched away to leave a thin InGaAs layer on top of the silicon wafer. Portions of the silicon substrate wafer are used as the multiplication region while the InGaAs layer is used as the absorption region. Mesa type device structures are formed to limit the active area of the device. Electrical contacts are formed at the top of the InGaAs mesa and at the bottom of the silicon substrate. In the operation of an APD, a reverse bias voltage is applied across these contacts. The reverse bias voltage creates electric fields between the contacts across the multiplication and the absorption regions.
For the proper operation of these composite InGaAs/silicon APD devices, it is essential that the electric field in the InGaAs absorption regions and the silicon multiplication regions be well controlled. The two regions have contradictory electric field requirements for their proper operation. For an InGaAs/silicon APD to operate at high frequencies, the electric field in its InGaAs absorption region should be sufficiently high to impart high velocities to the photogenerated charge carriers, but should be smaller than about 150 kV/cm to suppress carrier tunneling in the InGaAs material. However, the electric field in the silicon multiplication region preferably should be above 300 kV/cm for efficient avalanche multiplication.
In an attempt to separately set the electric fields in the absorption region and the multiplication region, the prior art composite APDs use a scheme in which a thin p+ doped silicon layer separates the two regions. The p+ layer may be a few nanometers thick. The thin p+ layer may be formed, for example, by shallow ion-implantation techniques. A portion of the reverse bias voltage applied to an APD is used in depleting the p+ layer. This reduces the voltage drop across the absorption region itself and may allow the electric fields in the absorption region to be set within a different range of values than the range of electric field values in the silicon multiplication region.
However, for this scheme to work the specifications on thickness and doping concentration of the p+ doped silicon layer are stringent. If either the doping concentration or the thickness of the p+ layer is on the high side, a larger portion of the applied reverse bias voltage is used to deplete the p+ layer. The consequently smaller voltage drop across the absorption region results in low values of electric field in the absorption region. These low values of electric field impair the bandwidth performance of the device. The reverse bias voltage required to completely deplete the p+ layer may even exceed the breakdown voltage of the device if either the doping or the thickness of the p+ layer is excessively on the high side. Similarly, if either the doping or the thickness of the p+ layer is on the low side, a smaller portion of the applied reverse bias voltage is used to deplete the p+ layer. Consequently, the p+ layer may not prevent the electric fields in the InGaAs layer from attaining high values at which the undesirable carrier tunneling occurs.
Unfortunately, the thicknesses and doping concentrations of thin implanted p+ layers are parameters that are susceptible to fabrication process variations. Conventional shallow ion-implantation techniques are not yet sufficiently developed to reliably or reproducibly control these parameters.
It would therefore be desirable to have composite APD structures in which the electric fields do not depend on sensitive process parameters and which can be fabricated with a wider processing latitude.
Another disadvantage of prior-art composite APD structures is that the electric field values in the absorption regions also are sensitive to changes in the applied reverse bias voltage. In APDs with p+ doped layers, the electric fields are at a maximum at the edges of the absorption regions. With increasing reverse bias voltages the electric fields increasingly penetrate into the multiplication regions in a direction away from the absorption layers. This feature of the APD structures with p+ doped layers is not conducive to controlling electric fields at high values within the absorption regions. Slight changes in the reverse bias voltages may cause large changes in the electric fields in the absorption regions. It is therefore further desirable to have composite APD structures in which the electric fields in the absorption regions change gradually in response to changes in the applied reverse bias voltages.
The invention is directed toward composite APD devices in which the multiplication regions and absorption regions are made of different semiconductor materials. The APD device structures of the present invention are designed to achieve control of electric fields in the absorption regions and the multiplication regions without use of intervening p+ doped layers. The composite APD devices may be fabricated using wafer fusion, bonding or any other suitable techniques for making composite structures.
Control over the electrical field values in an APD device is obtained by placing the p-n junction diode that is used to separate charge carriers in the APD device at some suitable distance away from the absorption region in the device structure. The electrical fields values in the absorption region and the multiplication region may be controllably chosen for high gain-bandwidth device performance.
In one aspect the present invention is directed toward APD device structures which are formed on a compound semiconductor substrate wafer, for example, an InP wafer. An epitaxial absorption layer (e.g., InGaAs) is grown on the InP wafer substrate. Portions of this layer serve as the absorption regions of the APD devices. Using wafer fusion, bonding, etching, thinning and other suitable techniques a thinned silicon layer is disposed on the top of the epitaxial absorption layer. The thickness of this thinned silicon layer may be controlled, for example, using iterative measurement and etching techniques. Portions of this silicon layer serve to form the multiplication regions of the devices. The silicon layer may, for example, be pxe2x88x92 doped. By implanting or diffusing, for example, n-type dopants, through the top surface of the silicon layer, n or n+ doped surface regions are formed in the silicon layer. These n-doped surface regions and the underlying pxe2x88x92 doped silicon layer make up p-n junction diodes which serve to separate charge carriers during the operation of the APD devices. Conventional metallization schemes may be used to make metal contacts on the top surface of the silicon layer and to the backside of the InP substrate.
The thinned silicon layer/InGaAs composite supported on the InP substrate may be processed further to form APD devices and circuits. For example, suitable ion-implanted guard ring structures may be formed to define active device areas and to form planar APD devices. Additionally, electronic components, such as JFETs or HBTs may be formed on the InGaAs layer and the InP substrate adjoining individual planar APDs devices to form monolithic transimpedance photoreceivers. For some high frequency applications for which APD active areas must be well defined, mesa-type devices may be formed using conventional patterning and etching techniques to etch through the silicon and InGaAs layers.
In another aspect the present invention is directed toward APD device structures formed on a silicon substrate wafer. These device structures may, for example, use mesas to define the active device areas. A pxe2x88x92 doped epitaxial silicon layer is formed on an n+ doped substrate wafer. The pxe2x88x92 doped silicon layer serves as the multiplication region of the APD devices. Using techniques such as wafer bonding and substrate etching, an nxe2x88x92 doped InGaAs absorption layer is disposed on top of the pxe2x88x92 doped epitaxial silicon layer. Contact to the InGaAs layer may be made using conventional multi-layer structures such as a p+ GaInAsP contact layer grown on suitable p-type InP and graded InP buffer layers which are first grown on the InGaAs layer. Alternatively, contact to the InGaAs layer may be made through a p+ InGaAs layer grown directly on the InGaAs layer. Common patterning and etching techniques may be used to etch device mesas. These mesas may have any diameter suitable for optical coupling to optical fibers. Metallic layers disposed on the back side of the silicon substrate wafer and on the top of the InGaAs contact layer form terminal contacts for applying signals and bias voltages to the APD device.
The APD device structures of the present invention whether of the type formed on InP substrates or of the type formed on silicon substrates do not depend on the use of a p+ doped layer to control the electrical fields in the absorption and the multiplication regions of the devices. In APD devices of the present invention, the electric field maximum under reverse bias operation conditions is located at the p-n junction in the multiplication region a distance away from the absorption region. In the case of devices formed on a silicon substrate the p-n junction is at the interface between the pxe2x88x92 epitaxial silicon layer and the n+ silicon substrate. In the case of devices formed on an InP substrate the p-n junction is at the interface between the thinned pxe2x88x92 silicon layer and the n or n+ doped surface regions. As the reverse bias voltage applied to an APD increases the electric fields in the device increase and move toward the InGaAs absorption layer. Since the diode p-n junctions are located at a distance, (for example, equal to the thickness of the silicon epilayer or the thickness of a thinned silicon layer) away from the absorption InGaAs layer, the electric field increases in absorption region gradually and controllably. The electric field values in the absorption region depend primarily on the thickness and doping concentration of the silicon layer (epitaxial or thinned). These parameters are well controlled, for example, in conventional epitaxial silicon growth processes.
By suitable choice of dopant concentration and silicon layer thickness and the electric field in the absorption layers may be set at a high value which corresponds to high carrier velocities in the absorption region, but yet which is below electric field values which cause carrier tunneling in InGaAs. The APDs of the present invention can, therefore, operate with a higher response speed and a larger bandwidth compared to prior art devices.
Further features and advantages of the present invention will be more apparent from the accompanying drawings and the following detailed description.